1. Field of the Invention
This invention relates to an apparatus for optical beam shaping and diffusing, a method for making the apparatus, and a method for using the apparatus.
2. Related Art
The ability to fabricate integrated circuit chips with increasingly smaller feature sizes depends upon continual evolution of photolithographic methods. Typically, a light source is used to illuminate a mask (reticle) so that a pattern is transferred into photoresist applied to an underlying semiconductor wafer. Machines that performs this operation are referred to as wafer steppers or wafer scanners. In order to achieve an accurate representation of the reticle pattern at submicron dimensions on the photoresist, it is necessary to use a light source that can support both a high degree of resolution and depth of focus. This requirement has led to the use of lasers as light sources for photolithographic applications.
However, the use of laser light for photolithography is not without its drawbacks. The pursuit of ever smaller feature sizes has focused semiconductor industry efforts on reducing the wavelength, rather than on boosting the penetrating power, of the impinging light. Therefore, state-of-the-art photolithographic processes now depend on wavelengths in the deep ultraviolet and far ultraviolet ranges. At these wavelengths, optical elements made of traditional amorphous materials, such as glass, cannot support optical transmission. Hence, optical elements in an optical train now must be made of crystalline materials that can support optical transmission.
Additionally, a cross section of a beam of light emitted from a laser source usually has an elliptical or substantially rectangular shape. Furthermore, an intensity profile across the cross section typically resembles a Gaussian distribution. Also, the rate at which the intensity tapers off (from its peak value near the center of the cross section) in one dimension can be different from the rate at which it decreases in an orthogonal dimension. Left uncorrected, the combined action of these various attributes can effect the degree of exposure of the photoresist which, in turn, can cause variations in linewidth dimensions of photolithographic patterns and the resulting electronic elements formed on the semiconductor wafer. Where these variations in linewidth dimensions are such that there is a difference between linewidth dimensions for horizontal lines and linewidth dimensions for vertical lines, the condition is referred to as horizontal-vertical (H-V) bias. Because H-V bias can effect the performance of an integrated circuit, methods for improving the control of variations in linewidth dimensions have been the subject of assorted efforts.
U.S. Pat. No. 5,962,195 teaches a method that uses two layers of photoresist, two exposure steps, and two etching steps. A light source illuminates a mask (reticle) so that a pattern is transferred into the topmost photoresist layer. The topmost photoresist layer is then etched by a first method to form a mask for an underlying photoresist layer. The underlying photoresist layer is resistant to the first etch method. Next, the light source illuminates the photoresist mask so that a pattern is transferred into the underlying photoresist layer. The underlying photoresist layer is then etched by a second method to pattern electronic elements onto a semiconductor wafer. This two-exposure, two-etch process acts to improve control of variations in linewidth dimensions of the electronic elements patterned on the semiconductor wafer.
U.S. Pat. No. 6,021,009 describes a technique for controlling linewidth dimensions directed towards homogenizing an intensity profile of an optical beam used for photolithography. The technique taught, however, realizes homogenization by absorbing light, with an optical compensator, at the areas of peak intensity within the intensity profile, thus reducing an overall intensity of the optical beam.
In a more general sense, the ability to shape a cross section of an optical beam or alter its intensity profile can be desirable for a variety of optics applications including, but not limited to, material ablation, micromachining, corneal sculpting, microsurgery, microbiology research, optical actuators, optical networks, and optical communications.
Several techniques have been developed for shaping a cross section of an optical beam.
U.S. Pat. No. 5,553,174 discloses a cylindrical minilens placed in an optical train after a collimating lens. The collimating lens receives an optical beam and collimates it along one dimension. The cylindrical minilens receives the optical beam from the collimating lens, collimates it along an orthogonal dimension, and expands or reduces it along the orthogonal dimension to shape the cross section of the optical beam.
U.S. Pat. Nos. 5,663,980 and 6,014,361 both teach an optical train that uses a prism to shape a cross section of an optical beam.
U.S. Pat. No. 6,075,650 describes a design for a single lens that can receive a diverging optical beam and circularize a cross section of it. The design includes parameters to compensate for misalignments between the source of the optical beam and the lens. In alternative embodiments, the lens can be designed to collimate the optical beam, cause it to converge, or cause it to diverge. No effect on an intensity profile of the cross section of the optical beam is disclosed.
However, in other documents, various methods are taught to modify an intensity profile of a cross section of an optical beam.
U.S. Pat. Nos. 5,610,733, 5,850,300, 6,025,938, and 6,075,627 all disclose beam homogenizers. The beam homogenizers are realized as a pattern of diffractive gratings such that portions of an incident optical beam, that emerge from the pattern of diffractive gratings, diffract and overlap with one another in a manner that equalizes an intensity profile of a cross section of the optical beam. The pattern of diffractive gratings can be formed using photolithographic, etching, electron-beam writing, or other techniques. Fourier Transform analysis can be used to determine the pattern of diffractive gratings. U.S. Pat. No. 6,075,627 superimposes its pattern of diffractive gratings on a lens pattern to form an integrated optical element. The lens pattern is used to focus or deflect portions of the incident optical beam that pass through it. However, no effect on changing the shape of the cross section of the optical beam is disclosed.
U.S. Pat. No. 6,069,739 teaches a method for making an intensity profile of a cross section of an optical beam more uniform by imparting time delays in portions of the cross section of the optical beam to reduce temporal coherence across the cross section. A two-dimensional array of fly's eye lenses is placed in the path of the optical beam. Each fly's eye lens acts on the portion of the optical beam that passes through it to disperse it. Different time delays are imparted into the various portions of the optical beam to reduce further the temporal coherence across the cross section. The time delays are realized by forming some of the fly's eye lenses to include, along an optical path, a second material that acts to change the speed of light in the portion of the optical beam that passes through it. Alternatively, each of the fly's eye lenses can be formed of an identical material and the time delays can be realized by using one or more prisms placed along the optical path before the array of fly's eye lenses. The various portions of the optical beam traverse distances of different lengths between the prism and their respective fly's eye lens, thus imparting different time delays.
None of these teachings disclose a single apparatus for use in both beam shaping and intensity profile modification. What is needed is an apparatus that can shape a cross section of an optical beam, and alter its intensity profile while minimizing a reduction in an overall intensity of the optical beam. Advantageously, such an apparatus should be inherently robust in design, insensitive to minor misalignments along the optical path, simple to manufacture, and inexpensive.